The objectives of the proposed research are two-fold: (1) to characterize the gene expression supporting the preimplantation development of the mammalian embryo; and (2) to identify the major, hormonally-regulated factors which influence the rate of protein synthesis in the mammalian blastocyst. The work related to the first objective will involve RNA/DNA hybridizations under conditions where RNA is present in excess and will be designed to answer questions regarding the complexity and stage-specificity of transcripts from single-copy DNA in the rabbit oocyte and preimplantation embryo. Single-copy DNA sequences will be prepared by hydroxylapatite chromatography and iodinated to high specific activity. RNA will be extracted from whole embryonic cells or sub-cellular fractions and used to determine the proportion of single-copy DNA represented in the transcripts recovered from various early developmental stages. Transcripts from different stages will be compared for similarity and divergence, either by joint hybridization (additivity) or by preparing a DNA probe homologous to one set of transcripts and then assaying its homology with another set. Work related to the second objective will focus chiefly on those factors which influence the initiation of polypeptide synthesis. The RNA components of embryonic polysomes will be radiolabeled in vitro under conditions in which embryonic growth is occurring at near-normal rates. The polysome profiles obtained from these embryos will be analyzed by sucrose gradient sedimentation and will serve as controls for those exposed to varying subsequent test conditions. In particular, the effects of variations in the concentrations of several ions as well as free sulfhydryl groups on polysome assembly and peptide chain initiation will be tested. The stability of the components during periods of polysome disaggregation will also be tested.